The present invention relates to a pattern forming method of forming a pattern on a resist using light exposing and electron beam (EB) exposing.
Photolithography has advantages such as simple processes and low cost and therefore is popularly used in the manufacture of semiconductor devices. Recent development of a shorter-wavelength light source (KrF excimer laser source) has made manufacture of devices with 0.25 .mu.m geometries or less possible.
To achieve further shrinkage in device geometries, an ArF excimer laser source with an ultrashort wavelength and a phase shift mask (Levenson type etc.) have been examined. These are expected as mass-production lithography tools corresponding to the 0.15-.mu.m rule. However, putting these tools into practical use poses a lot of problems and still requires a long time. Putting the tools into practical use may delay and may not reach the market-required level for some time.
Electron beam lithography is the first candidate of post-photolithography techniques. In electron beam lithography, an electron beam is focused and scanned on a resist, thereby forming a pattern on the resist. Electron beam lithography has reportedly achieved micropatterning on the order of 0.01 .mu.m.
Electron beam lithography, however, has a problem of low throughput. That is, the number of wafers processed per hour is small. In electron beam lithography, a pattern is formed on a resist by two-dimensionally scanning the resist with a thin electron beam. For this reason, the throughput becomes lower than that of photolithography wherein a pattern is fully transferred.
To increase the throughput, a cell projection scheme for repeatedly forming the same pattern portion in a ULSI pattern has been developed. However, even the cell projection scheme has a much lower throughput than that of photolithography.
As a method of improving the throughput of electron beam lithography, mix and match has been proposed. In mix and match, light exposing and EB exposing are used to transfer a pattern to a resist. In this mix and match method, light exposing is used instead of EB exposing as far as it is possible, thereby improving the throughput.
However, the mix and match lithography technique suffers a problem that a pattern formed by light exposing is blurred by back scattered electrons. To solve this problem, complex data processing is required to design pattern data smaller than the actual pattern by any dimensional error expected to result from back scattered electrons.
A method using a phase shift mask is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-155812. In this prior art, most pattern portions are transferred using a phase shift mask by light exposing, and only portions which have defects due to the phase shifter arrangement are corrected by an electron beam. With this process, the number of regions formed by the electron beam is minimized to improve the throughput. This method allows to reduce the number of regions formed. However, since a pattern having a resolution lower than the limit resolution of the phase shift mask cannot be transferred, this method cannot cope with device micropatterning in the future.
To manufacture a small number of devices of various types, it takes time to prepare masks. As a means for solving this problem, Jpn. Pat. Appln. KOKAI Publication No. 1-293616 discloses a method of transferring a common pattern to a resist by light exposing while forming a non-common portion using an electron beam. With this method, the number of masks can be small, so the mask preparation time can be shortened.
However, this method cannot cope with a case wherein a pattern having a resolution lower than the resolution of light used is present, like the above-described method using a phase shift mask. In addition, patterns to be formed by an electron beam are interconnection portions or the like. To expose such pattern using an electron beam, the pattern is formed by sequentially delineating one-dimensional segments, and this requires a long time. Hence, this method can hardly be applied to a lithography system for forming a fine pattern at high speed.
As described above, in the conventional mix and match method using light and electron beam for the same layer, which has been performed to improve the throughput, the resolution of EB exposing cannot be sufficiently exploited, and the throughput is lower than that of light exposing.
To solve the above problems, a lithography system capable of obtaining the resolution of an electron beam and the same throughput as that of a stepper has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 9-46683). In this lithography system, a coating/developing equipment applies a resist on a wafer. The wafer with the resist is conveyed from the coating/developing equipment to a light stepper. In the light stepper, the resist is exposed by light. With this operation, a rough pattern is transferred to the resist. Next, the wafer is conveyed from the light stepper to an EB exposure equipment. In the EB exposure equipment, the resist is exposed by an electron beam. With this operation, a fine pattern is formed on the resist. In this process, cell projection is employed to improve the throughput.
As is known, the throughput of EB exposing is lower than that of light exposing. For this reason, a plurality of EB exposure equipments are arranged for one light stepper. Wafers processed by the light stepper are parallelly processed by the plurality of EB exposure equipments. After a rough pattern and fine pattern are formed, the wafer is conveyed to the coating/developing equipment and developed. With this process, the pattern is formed.
The resist usable in such a lithography system is a chemically amplified resist such as UV2HS or UVN-HS (available from Shiplay) with a high resolution and high sensitivity. Since the chemically amplified resist is modified to produce dimensional errors in the presence of various chemical substances in the air, the environment during conveyance must be controlled.
When such a lithography system is constructed, a device pattern including a fine pattern based on the 0.1-.mu.m rule can be formed at a high throughput.
As described above in detail, a lithography system which forms a rough pattern by light exposing and a fine pattern by EB exposing acquires a high resolution of EB exposing and a high throughput of the light stepper.
However, the lithography system has the following problems. The lithography process is roughly classified into three steps: light exposing, EB exposing, and heating. The chemically amplified resist employed in this system consists of a polymeric material. The exposing process is classified into two stages. At the first stage, the resist portion irradiated with light or an electron beam absorbs the energy. A negative resist causes a crosslinking reaction to be insoluble while a positive resist causes a decomposition reaction to be readily soluble. At this first stage, an acid is generated in the resist in response to light or electron beam irradiation. Exposure at the next stage takes place when the polymeric material causes a crosslinking reaction (negative resist) or a decomposition reaction (positive resist) using the acid as a catalyst.
The acid diffuses or is deactivated in the resist during the interval from exposure to heating. This diffusion or deactivation generates a dimensional error, i.e., excessively thins or thickens the exposure pattern.
To reduce the dimensional error, the interval from exposure to heating need be shortened. However, in the above system, since the light stepper exposes the resist in the air while the EB exposure equipment exposes the resist in a vacuum atmosphere, not only the conveyance time from the light stepper to the EB exposure equipment and the EB exposing time but also an exhaust time need always be inserted between light exposing and EB exposing to change the environment from the atmospheric pressure state to the vacuum state. Therefore, the time from light exposing to heating cannot be largely shortened.
In addition, in this lithography system, several EB exposure equipments must be used to ensure a throughput suitable for a mass-production tool, resulting in a bulky system.